Reactivity of Sulfhydryl Groups in Micelles

In order to get some insight into the factors determining the different reactivities of protein SH groups, the reactions of a micelle‐forming cysteine derivative, the N ‐dodecanoyl‐DL‐cysteinate anion, with iodoacetamide, chloroacetamide, and p ‐nitrophenyl acetate have been studied kinetically in aqueous solution at 25°. The incorporation of the mercapto compound into an anionic micelle, consisting of N ‐dodecanoyl‐DL‐cysteinate alone or in mixture with N ‐dodecanoyl‐glycinate, is accompanied by a “masking” of the sulfhydryl group. On the other hand, when the model compound is incorporated into a cationic micelle, formed by hexadecyltrimethyl‐ammonium, bromide, the true second‐order rate constants for the reactions of the RS − species with chloroacetamide, iodoacetamide, and p ‐nitrophenyl acetate increase by factors of 5–7, 60–100, and 100–200, respectively, in comparison to the corresponding constants in the bulk phase. The observed effects on the apparent second‐order rate constants related to the total concentration of mercapto compound are even higher, since the degree of SH ionization is greater in the eationic micelle than in the bulk phase. The magnitude of the latter effect decreases strongly with addition of other electrolytes (NaBr) to the medium, due to a decrease of the surface potential of the micelle. The reactions of N ‐acetyl‐L‐cysteinate and L‐cysteine with iodoacetamide and p ‐nitrophenyl acetate are also strongly accelerated in the presence of cationic micelles under the condition that the test solution contains no or only small amounts of other electrolytes. It is assumed that the catalytic effect of the cationic micelle is mainly caused by the participation of a positively charged head group spatially close to the RS − group in the activated complexes of the reactions. The “masking” of the SH group in the anionic micelle could also be related, at least in part, to electrostatic effects. In agreement with this assumption is the fact, that the addition of a nonionic micelle‐forming surfactant, Brij 35 (polyoxyethylene dodecyl ether), has only a relatively small effect on the reaction rates.